This invention relates to high resolution television as disclosed in U.S. application Ser. No. 06/515,220 filed July 19, 1983, a continuation-in-part of application Ser. No. 06/364,884 filed Apr. 2, 1982, and now abandoned and more particularly to an improvement in technique for achieving high resolution television within a bandwidth limited transmission channel.
In the United States, the National Television Systems Committee (NTSC) adopted certain standards for color television transmission using a technique that so encodes color information in the composite video signal that the color information will not detract from the ability of a monochrome receiver to display the image in black and white, while color receivers can use the encoded color information for display of the image in full color. The NTSC system, based on the utilization of three color primaries (red, green and blue) has been adopted by other countries for use within their prescribed standard for monochrome transmission to black-and-white television receivers. Consequently, it should be understood that this invention to be described, with reference to NTSC standards, is applicable by analogy to other systems designed to different standards, such as PAL and SECAM.
In accordance with the invention disclosed in the aforesaid application, vertical resolution is increased for standard television by modulating the vertical scan axis of a television camera such that each active scanning line traverses an undulatory path rather than a straight line, i.e., oscillates about the normal horizontal scan line. The amplitude of the modulation is preferably sufficient for interlaced lines of a frame to intrude the area of adjacent lines, which is modulation with an amplitude equal to plus and minus one half line spacing of the interlaced fields above and below the center of the normal scan line. The video signal thus produced by actually scanning areas above and below the normal scan lines is then transmitted just as for the normal scanning lines, which preferably would be within a wider bandwidth of about 8 MHz or more, but can be within the vestigial bandwidth limit of about 4 MHz for NTSC color transmission through a 6 MHz channel.
At a monitor or television receiver, a local oscillator modulates the vertical scan axis of each line to be at the same frequency, phase and relative amplitude as the modulation employed in the television camera in order to synchronously reproduce each line of a frame. The local oscillator may be synchronized with the television camera by information contained in the received video signal. For example, in NTSC color transmission, a gated burst of the color subcarrier is transmitted during horizontal blanking periods for use in regenerating the color subcarrier as a reference in the monitor or television receiver, hereinafter referred to generally as the display unit. Consequently, a convenient modulation frequency is the fundamental or preferably some even harmonic of the chrominance subcarrier frequency (3.579545 MHz in the NTSC system). If the modulation in the television camera and the display unit are synchronized, the display unit will reproduce the frame in the same manner as it was scanned in the camera, which is with increased vertical resolution.
The second harmonic (7.15909 MHz) of the color subcarrier frequency, or other even harmonic, such as some multiple of the second harmonic of the color subcarrier frequency, is preferred over the fundamental (3.579545 MHz) because, at the standard 15.734 kHz line scan frequency, each line will have substantially a whole number of modulation cycles, such as 455 for the second harmonic, rather than a fractional number of modulations, namely 227.5 for the fundamental, thus making it easier to maintain the same modulation phase from line to line in a frame.
Modulation of the scan line with a higher frequency than the fundamental chrominance subcarrier will increase the bandwidth requirements of the color subcarrier side bands, usually between 3.75 MHz and 4.2 MHz. For standard transmission through a 6 MHz channel, such as a broadcast channel allocated by the Federal Communications Commission, the color subcarrier sidebands must be limited by a filter. While vertical resolution is increased even with a limited transmission channel (from significantly less than 485 active lines in a frame of 525 scan lines to significantly more, such as about the equivalent of 600 scan lines), the full potential of increasing vertical resolution to the equivalent of 800 scan lines per frame, or more, will not be realized because of the color subcarrier sideband filter. While it may be possible to omit the filter in some special transmission networks, such as private cable networks, it would be desirable to achieve the full potential of high resolution television for standard transmission.